The present invention relates to quartz glass-made optical fibers coated with a silicone composition and a method for the preparation thereof or, more particularly, to silicone-coated, quartz glass-made optical fibers from which the coating layer of cured rubbery silicone is readily peelable to facilitate the connecting works for making joints between the terminals of the optical fibers in extending the lines.
As is known, the technology of communication by use of optical fibers is under rapid growing in which, although optical fibers can be made of various transparent materials including fused quartz glass, multicomponent glasses and synthetic plastic resins and the like, most of the practically and currently employed optical fibers are made of fused quartz glass in view of the lightweight, low transmission loss, absence of induction and heat and weathering resistance as well as the large transmission capacity thereof.
Since a quartz glass-made optical fiber is usually very slender having a diameter of only a fraction of a millimeter and hence fragile and is also subject to contamination with external stainas well as deterioration by moisture, it is a generally undertaken way that quartz glass-made optical fibers are provided on the surface with a protective coating which is usually formed by first coating the fiber with a primary coating composition capable of being converted into a rubbery layer and an overcoating is provided thereon with a coating material having larger toughness.
The coating material used for the primary coating should satisfy various requirements such as low temperature-dependency of the rigidity, versatility in a wide temperature range of use, effectiveness for mechanical reinforcement and stress relaxation, low transmission loss even in bending of the fiber with a small radius of curvature and little tendency to cause noise by light scattering. In this regard, several types of silicone compositions are widely accepted for the purpose as the most suitable ones among a variety of polymeric coating materials.
In practice, conventionally used silicone materials for providing coating on quartz glass-made optical fibers are limited to a few types and the method for providing coating is accordingly limited (see, for example, Japanes Patent Publication No. 56-11122 and -11123 and U.S. Pat. Nos. 3,980,390 and 4,270,840). For example, a composition of a thermally curable silicone admixed with an organic peroxide is applied on to the surface of the optical fibers followed by curing with heating. Alternatively, curing of a silicone composition is performed by heating a mixture comprising an organopolysiloxane having silicon-bonded vinyl groups in the molecule and an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms as catalyzed by a platinum catalyst to cause addition reaction of so-called hydrosilation. One of the problems in these methods is that the curing must be effected with heating in an oven kept at a high temperature according to the types of the silicone so that uniformity in curing can hardly be ensured in the absence of very accurate control of the oven temperature in addition to the inherent disadvantage that the velocity of curing in these methods is limited. Therefore, these methods are not suitable for high-speed production of optical fibers. Moreover, high-temperature curing of the silicone coating composition necessarily causes sticking of the coating layer to the surface of quartz glass so that peeling of the coating layer can be performed with great difficulties as is required when extension of the communication lines is desired by making a joint between terminals of the optical fibers.